1. Field of the Invention
The present invention relates to measuring earth chords in a satellite positioning system and, more particularly, to measuring long earth chords or measuring earth chords in a satellite spinning at a low spin rate.
2. Description of Related Art
Once a satellite is separated from a rocket launch vehicle, rocket thrusters on the satellite are activated to place the satellite into a required orbit, such as a geosynchronous orbit around the earth. In order to determine when and in what direction the thrusters should be activated to reach geosynchronous orbit, the location and attitude of the satellite in relation to the earth must be determined. Because the orientation of the satellite is not typically seen or measured from the earth, the attitude of the satellite is normally determined through use of on-board sensors on the satellite. The on-board sensors send, by telemetry, information to a ground station on the earth, which processes the information to determine the orientation of the satellite and fire the thrusters accordingly.
Generally, a satellite includes an earth chord sensor, which is an infrared sensitive device that detects the heat of the earth as the satellite spins and sweeps the sensor across the earth. One such earth chord sensor is the Barnes Horizon Crossing Indicator (HCI), model no. 283401-1001-9, manufactured by Barnes. Ideally, this earth chord sensor generates a near trapezoidal pulse, wherein the leading edge of the pulse indicates when the sensor has first contacted the earth and the trailing edge of the pulse indicates that the sensor has completed its sweep across the earth. The width of the pulse indicates the earth chord. For example, if the sensor sweeps across the equator, the pulse would be much wider than if the sensor sweeps across an area near the earth's poles.
FIG. 1 is a block diagram of a prior art system for processing the output of an earth chord sensor on a satellite. The earth chord sensor 10 receives infrared radiance and converts the thermal input into an electrical signal which is conditioned by a amplifier/compensator 12 to produce a pulse. If the satellite is spinning at a relatively high spin rate (i.e. about 10 rpm) or if the earth chord being measured is short (i.e. about 1-2 seconds), the output of the amplifier/compensator 12 is an inverted near trapezoidal pulse, as indicated by block 14 on FIG. 1. The pulse is passed through an inverter 16 which reverses the polarity of the pulse. The output pulse from the inverter 16 is provided to an analog threshold circuit 18 which compares the pulse to a predetermined voltage threshold (for example, 1 volt). A time-of-arrival (TOA) logic circuit 20 then calculates the time at which the leading edge and trailing edge of the pulse cross the voltage threshold. All of these components may be included in hardware on the satellite.
The leading edge and trailing edge times calculated by the TOA logic circuit 20 are then provided to a TOA software processor 22 which simply subtracts the two times to calculate the time it took the sensor 10 to sweep across the earth. The TOA software processor 22 may be included as part of a central spacecraft processor (SCP) located on the processor. The earth chord time is then transmitted by telemetry to a ground station on the earth, which computes the orientation of the satellite. The ground station may also receive information from other on-board sensors, such as a sun sensor, that is used in computing the orientation of the satellite.
This prior art system effectively calculates the earth chord time as long as the satellite is spinning at a high enough rate or the earth chord is short enough for the sensor to generate a near trapezoidal pulse. If the satellite is spinning at a lower rate (i.e. less than about 10 rpm) or the earth chord is long (i.e. greater than about 3 seconds), however, the earth chord sensor generates an irregular pulse 24 as shown in FIG. 2. At low spin rates or with long earth chords, when the earth chord sensor first encounters the earth, the sensor generates a sharp negative voltage peak (referred to as the leading edge peak). As the sensor sweeps across the earth, the sensor "droops" upwards to a positive voltage. When the earth leaves the view of the sensor, the sensor produces a sharp positive voltage peak (referred to as the trailing edge peak) and then slowly "droops" towards a zero voltage. The droop in the sensor output at low spin rates or during long earth chords is caused by the sensor behaving like a high pass filter.
As shown in FIG. 2, the true earth chord time for the low spin rate pulse 24 is the time between the leading edge peak and the trailing edge peak. The prior art system of FIG. 1, however, which calculates the earth chord time by determining the leading edge and trailing edge of a pulse at a threshold voltage level, incorrectly calculates the earth chord time of the low spin rate pulse 24 as the width of the leading edge pulse at the voltage threshold. Therefore, there remains a need for a system and method for determining earth chord times for a satellite spinning at a low spin rate (i.e. less than about 10 rpm) or which is measuring a long earth chord (i.e. greater than about 3 seconds).